Control valves are used to control the flow of fluids in many processes. They can be used to control the flow of raw materials and finished product to and from process vessels; they can be used to control the flow of hydraulic fluid in hydraulic control mechanisms; they can be used in a wide range of other applications including municipal water supplies and natural gas supplies, and chemical processes. Control valves operate by means of a control mechanism which moves a restricting device within a control valve housing. By restricting the flow through the control valve, a pressure drop occurs and the flow of fluid is reduced. In many applications it is desirable to have a control valve where no restriction occurs when the valve is in a full open position. Additionally, since the control valve is responsive to a control mechanism, it is desirable to have a control valve which is not overly sensitive to inputs from the control mechanism. (An overly sensitive control valve will give an undesirably large change in pressure drop in response to a small input from the control mechanism.) In many applications it is also desirable to be able to control the flow with a high precision, such as within 1% or less of the total flow range. Due to their inherent design, most prior art control valves of this type have inherent flow restrictions, even when the valve is in a full open position. Further, many of the prior art control valves are restricted to controlling the flow across a dimension no larger than the inlet or outlet diameter of the control valve which is directly related to the diameter of the respective pipe or tube through which the fluid flows to the valve. This limitation on the control distance has a direct impact on either the flow range or the precision to which flow may be controlled using the control valve.
It is therefore an object of this invention to produce a control valve having both high range and high precision.